1. Field of the Invention
This invention relates to subscriber loop carrier systems and, more particularly, to automatic number identification in digital subscriber loop transmission systems.
2. Description of the Prior Art
It is common practice on two party telephone loops to provide a direct current ground connection at the subset of one of the two subscribers but not at the other. When the subscriber with the ground connection goes off-hook, the ground connection can be detected at the central office (by current measurements) to insure proper billing to the calling subscriber. Detection is accomplished by monitoring the balanced longitudinal current to the subscriber subset.
In subscriber loop carrier systems where the connection between the subscriber and the central office includes carrier facilities, a direct measurement of loop current is no longer feasible. In this situation, it is necessary to encode the automatic number identification signals in such a fashion as to permit its transmission through the carrier system. The automatic number identification signal must then be detected at the central office location and repeated to the central office circuits to insure proper billing.
Since the metallic loop is not generally available on a continuous basis for ANI signals in a carrier system, another method for transmitting ANI information must be used. Moreover, changes in the calling party must be detected to bill the correct subscriber for a second call.